The present invention relates to a system and method for automatically controlling a running speed of a moving object such as a vehicle to a desired cruising speed. The present invention is particularly but not exclusively applicable to the system and method for automatically controlling the vehicle speed in which the vehicle speed is automatically controlled so as to coincide with a set cruising speed at which the vehicle driver desires to run.
Such automatic vehicle speed controlling systems are exemplified by a Japanese Patent Application First Publication (Tokkai) Sho 59-8023 published on Jan. 17, 1984.
In the above-identified Japanese Patent Application document, the automatic vehicle speed controlling system includes a vehicle speed sensor which senses an actual vehicle speed and outputs an operational data in proportion to the actual vehicle speed, a command switch group having a set switch for operatively outputting a cruise command signal, a memory for storing the operational data on the vehicle speed derived from the vehicle speed sensor in response to the cruise command signal of the set switch, an actuator responsive to a command signal for driving a throttle valve of a vehicular engine so that an angular position of the throttle valve is displaced, and a control unit for issuing the command to the actuator to drive the throttle valve according to a difference between the actual vehicle speed and stored set vehicle speed in the memory.
In the above-described automatic vehicle speed controlling system, when the set switch is operated to turn off after a turn-on operation of the set switch, the vehicle speed at the time of the set switch turn-off operation is stored in the memory. The control unit issues the command to the actuator according to the difference between the actual vehicle speed and stored vehicle speed. The angular displacement of the throttle valve by means of the actuator causes the vehicle speed to coincide with the stored vehicle speed. Thus, the vehicle is constantly run at the stored vehicle speed. Such automatic vehicle speed controlling systems are convenient for the vehicle to run on freeways.
Since engine output characteristics, gear reduction ratios and weights, etc., are individually different as are models of vehicles on which the above-described automatic vehicle speed controlling systems are mounted, control constants such as those used in initializations of vehicle speed control programs, a constant K, gain G (control equation of (K .times.e.alpha.+.epsilon.).times.G}, and those used at the time of a release operation of an accelerator pedal are varied so as to cope with the variations such as the vehicle model output characteristics.
However, since the constant K and gain G affect a control performance of the automatic vehicle speed controlling systems, the control over the vehicle speed is carried out with the same control constants (K and gain G) in the control equation as those immediately after the above-described given operation of the set switch in a transient state and as those during the cruising speed controlling a steady state. Therefore, although appropriate control constants (K and gain G) for each of the vehicles are set, overshooting or undershooting of the vehicle speed occurs for a long duration of time immediately after the off operation of the set switch and such unstable states cannot be settled depending on a road condition (e.g., ascending slope or descending slope) on which the vehicle runs.
In addition, in a case where an adjustment of a control wire connected between the actuator and throttle valve such as for tension or length, etc., is inappropriate even though a strictly set initialize value corresponding to the vehicle speed at the time of the off operation of the set switch is applied to the actuator, the overshooting or undershooting problem inevitably occurs at the transient state immediately after the off operation of the set switch.